Q.(i) Calculate the oxidation number of central metal atom of the following complex compound: [Cr(NH3)5Cl]SO4.

A coordination compound is first distinguished from a double salt by a solution test: a double salt (like Mohr's salt, FeSO₄.(NH₄)₂SO₄.6H₂O) dissociates completely into its simple constituent ions in water, while a coordination compound's complex ion (like [Fe(SCN)₆]³⁻ in potassium ferrithiocyanate) keeps its identity and does not break apart into simple ions.

Alfred Werner (1866-1919) proposed the first successful theory explaining coordination compounds in 1893, before the electron itself had even been discovered, purely from reaction-chemistry observations -- work that won him the 1913 Nobel Prize in Chemistry as the first inorganic chemist to receive it. His puzzle was the cobalt(III) chloride-ammonia system: CoCl₃.6NH₃, CoCl₃.5NH₃, and the cis/trans forms of CoCl₃.4NH₃ are four distinctly coloured compounds that precipitate different numbers of moles of AgCl per mole of complex, even though the ordinary valences of both starting materials are already fully satisfied. …